1. Field of the Invention
The present invention relates to a solid state imaging device, and more particularly relates to a solid state imaging device which comprises an optical black clamping circuit.
2. Discussion of the Related Art
From the past, as devices which form light from a photographic subject into an image, there have been known solid state imaging devices which utilize solid state imaging elements, such as charge coupled devices (hereinafter termed “CCD”s) and the like. In such a CCD, there is an optical black section (hereinafter “optical black” will be abbreviated as “OB”) in which light is prevented from falling, and thus does not perform photoelectric conversion. Such an OB section is disposed before or after the effective picture elements of the CCD. Since no light is incident upon this OB section, its output, i.e. the OB level, corresponds to a black level signal. In a solid state imaging device which utilizes such a CCD, this OB signal is used as a reference for the black level of the picture signal, and a clamping procedure is performed so as to match the OB level to the reference level.
FIG. 1 is an overall structural view showing a solid state imaging device which is an example of the prior art, while FIG. 2 is a block diagram of an OB clamping circuit incorporated therein. In FIG. 1, the output of a CCD 50 is inputted to an OB clamping circuit 52. In this OB clamping circuit 52, an OB section of the CCD 50 output is matched to a reference level by a signal OBCP 55 for OB clamping which is outputted from a timing signal generation circuit 53. After gamma and knee processing etc. have been performed by a signal processing circuit 54, it is thereafter outputted to the outside.
The operation of the OB clamping circuit 52 will now be explained with reference to FIG. 2. The output of the CCD 50 is inputted to the + side of an amplifier 56, and, after being amplified therein, is supplied via a switch 57 to a capacitor C11. The signal OBCP 55 is a control pulse, which performs closing action (turning ON) of a switch 57 at a timing corresponding to that of the OB section. When this is done, a voltage corresponding to that of the OB section of the CCD 50 output comes to be maintained in the capacitor C11.
Normally a dark current is included in the output of the CCD 50, and its values for the OB section and the effective picture element section are equal. Accordingly, the OB clamping operation, along with canceling the influence of the dark current, also performs the operation of adjusting the black level position.
The ends of a variable resistor VR1 are connected to the + and − sides of a voltage source Vcc, and the black level position is adjusted by adjustment of the variable resistor VR1. The output of the variable resistor VR1 is inputted to an amplifier 58 via a low pass filter (hereinafter termed an “LPF”), which is made up from a resistor R10 and a capacitor C10. In the amplifier 58, the level of the capacitor C11 and the output from the VR1 are compared, and their difference is amplified. Normally the open loop gain of an operational amplifier is about 100 dB (one hundred thousand times). In the amplifier 58, a gain of this level is attained as well. Accordingly, even if the potential difference between the − input and the + input of the amplifier 58 is very small, its output undergoes a large variation. Since the low pass filter is connected in the input of the amplifier 58, its output changes smoothly. The signal level of the output of the CCD 50 is adjusted by supplying the output of the amplifier 58 to the − input of the amplifier 56.
FIGS. 3 through 5 are timing charts for this solid state-imaging device according to the prior art. FIG. 3 is a horizontal timing chart. Within a single horizontal period (1H) of the output of the CCD 50, there are included an image period and an OB section for signal handling. In the period which corresponds to the OB section, the signal OBCP 55 goes H level, and the switch 57 closes. In other words, the shifted signal level over the entire horizontal period becomes the output of the OB clamping circuit 52, by maintaining the value of the period of the OB section in the capacitor C11.
FIG. 4 is a timing chart showing the vertical timing. In a single vertical period (1V), there is no signal output over the vertical blanking period, and moreover OBCP is not outputted. In normal OB clamping operation, there is no problem even if it is substituted by the voltage which is maintained in the capacitor C11 over the vertical blanking period (about 20 to 50 horizontal periods). In other words, the time constant based on the resistor R10 and the capacitor C10 is such a value as no affection.
FIG. 5 is a timing chart for four-field accumulation. This is operation in a high sensitivity mode when the reading out of the signal from the CCD 50 has been performed once in four. Since, if OB clamping operation is performed when there is no signal output from the CCD 50, an erroneous value will come to be retained, therefore the signal OBCP 55 is always maintained at L level in periods in which there is no output.
In FIG. 4, the level of the period of the OB section of the output of the CCD 50 is almost constant provided that there is no great change of the environment. Under this type of conditions the outputs of the amplifiers 56 and 58 are stabilized at constant values, since the amplifiers 56 and 58 constitute a closed loop. However, if observed rigorously, the output of the amplifier 58 has minute vibration.
This phenomenon will be explained in the following. FIG. 6 is a timing chart for the case of continuous clamping, while FIG. 7 is one for the case of n-field accumulation. In FIG. 6, the output of the amplifier 58 is shown enlarged upon the vertical axis. The signal OBCP 55 goes H level once in one horizontal period, in the period of the OB section, and its value is maintained in the capacitor C11. At this time the difference between the + input and the − input of the amplifier 58 is amplified according to open loop gain, and as a result the entire level of the output of the amplifier 56 comes to be shifted. After this, when the switch 57 closes again, the value of the OB section whose level has been shifted is inputted to the capacitor C11, and the output of the amplifier 58 also varies.
Accordingly, in FIG. 6, each time the signal OBCP 55 is outputted, the output of the amplifier 58 moves slightly up and down. The target value α is the value at which the output of the amplifier 58 really ought to stabilize, but actually the output of the amplifier 58 oscillates within a range of error ε about the value α as a center. In other words, to consider the entire circuit, when an error ε is detected from the target value α executed by the OB clamping, control is exerted in the direction to suppress this error ε.
Since the OB clamping operation is only executed once in a single horizontal period, the remaining period comes to be controlled at the value maintained at this time, and the error ε inevitably occurs. In other words, if the entire period were to be like the OB section (if the switch 57 were to be always turned ON), then the output of the amplifier 58 would reach the target value α. Since, in fact, the OB clamping action cannot be performed except by sampling control, therefore it is quite impossible to eliminate minute oscillations of the output of the amplifier 58.
However, the control method of FIG. 2 is one of the most excellent methods for reducing the error ε. The actual error ε is an extremely small value and from the point of view of the image signal it is a value which can be completely ignored.
However, in the case of this method, a problem arises when the accumulation field number is further increased. In the case of n-field accumulation shown in FIG. 7, the signal OBCP 55 is read out only once in n fields (the period T), while in other periods there is no output of the signal OBCP 55 (the period S). Accordingly, during n−1 fields, the value in the capacitor C11 undergoes almost no change.
As has also been explained with reference to FIG. 6, when at the end the signal OBCP 55 is outputted, the output of the amplifier 58 varies in the direction to suppress the difference from the target value α. Since the control for suppression of the error is not performed over the period S in which the signal OBCP 55 is not outputted, even if the target value α is exceeded, the error continues to increase. Finally it arrives at the power source voltage of the amplifier 58 and saturates, which is undesirable. Thereafter, by the period T starting and the signal OBCP 55 being outputted, the value in the capacitor C11 changes, and the output of the amplifier 58 changes towards the target value α.
However, the period T in which the signal OBCP 55 is outputted is only one vertical period, and when the period S starts without any feedback being performed the error from the target value is magnified as described above, which is undesirable. Since the variation in the output of the amplifier 58 shown in FIG. 7 is inputted to the amplifier 56 just as it is without alteration, this variation is also superimposed upon the input signal to the signal processing circuit 54. This appears as vertical shading upon the final picture signal, and entails a great breakdown of picture quality.
In particular, if as in FIG. 7 the output of the amplifier 58 reaches saturation level (±Vcc), then a great level shift occurs in the picture signal. Since this occurs alternately, a periodic white and black pattern comes to be repeatedly generated, and the same phenomenon occurs as oscillation. If the time constant is made large, this phenomenon can be more or less alleviated, but it is a phenomenon which inevitably occurs if the number of accumulation fields is increased, and it has become a problem which must be avoided at all costs.
Furthermore, if the signal OBCP 55 is regularly outputted including the period S, a signal, which is not outputted from the CCD 50, comes to be maintained in the capacitor C11. In other words, in this state, the normal feedback function does not operate. In this case, great shading is generated in portions where “signal present” changes to “signal absent”, or “signal absent” changes to “signal present”, and it reaches a level that cannot be restored by compensation processing. If the number of accumulation fields is further increased, the periodic white and black patterns described above may happen.